An aromatic polycarbonate (hereinafter may be abbreviated as “PC”) has excellent mechanical properties, electric properties, heat resistance, cold resistance and transparency and is a material which is used in various applications including lenses, optical disks such as compact disks, building materials, automotive parts, chassis for office automation equipment and camera bodies, and demand therefor has been increasing every year. The amount of PC products discarded increases along with an increase in demand for PCs, and most of them are dealt with by a method such as incineration or burying in the ground. Thus, the increase in demand for PCs accelerates not only depletion of oil resources but also deterioration of the global environment. Consequently, recycling of waste plastic has become important.
As a method for recycling waste plastic, (1) thermal recycling which comprises recovering waste plastic as thermal energy, (2) material recycling which comprises mixing waste plastic with new plastic or plastic of another kind in a given ratio and processes the plastic mixture into a product and (3) chemical recycling which comprises chemically decomposing waste plastic into a raw material for plastic and reusing it in production of plastic are primarily used. However, the thermal recycling essentially destroys the global environment and decreases resources since it produces carbon dioxide and water when burning plastic and recovering heat. The material recycling is environmentally desirable since it imposes the least burden on the environment in terms of consumption of resources. However, products which can be mixed with waste plastic are limited, the amount of waste plastic which can be mixed into a product is small, and the amount of waste plastic which can be recycled is limited. The chemical recycling is an industrially useful recycling method since it decomposes plastic into a raw material which can be directly used in production of plastic.
As a method of chemically recycling a PC, a method comprising decomposing the PC with an excessive alkali metal salt aqueous solution followed by neutralization to collect an aromatic dihydroxy compound has been known. In JP-B 40-016536, a PC and a 1 to 30% alkali aqueous solution are charged into a pressure-resistant container to hydrolyze the PC at 100° C. or higher, preferably 150° C. or higher, and the obtained hydrolysate is made acidic, dissolved in methanol and subjected to an activated carbon treatment to remove coloring components followed by reprecipitation to obtain white bisphenol. JP-A 54-048869 discloses a method comprising saponifying polycarbonate scraps in bulk or as a solution, separating unsaponified components, phosgenating the saponified mixture and using the resulting product in a polycarbonate polymerization process totally without purification and treatment processes. JP-A 6-056985 discloses a method comprising decomposing a PC with phenol in the presence of an alkali catalyst to collect an aromatic dihydroxy compound and a diaryl carbonate. Further, JP-A 10-259151 discloses a method comprising carrying out a transesterification reaction in a toluene, xylene, benzene or dioxane solvent by use of a small amount of an alkali as a catalyst to obtain a dialkyl carbonate and an aromatic dihydroxy compound. Further, JP-A 2002-212335 proposes a method comprising carrying out transesterification of PC and lower alcohol in the presence of a solvent such as alkyl chloride, ether or aromatic hydrocarbon solvent and a tertiary amine as a catalyst to obtain an aromatic dihydroxy compound and a dialkyl carbonate.
However, since the method disclosed in JP-B 40-016536 uses the thin alkali aqueous solution, the reaction takes place at high temperatures. Further, since the method uses a very large amount of water in a posttreatment to reprecipitate a yellow coloring component from methanol/water, liquid waste disposal is very complicated. The method disclosed in JP-A 54-048869 uses polycarbonate scraps in a polymerization reaction without a purification process, so that additives, colorants and the like which are used in plastic as nearly essential components are included in the PC production process and affect product quality. The methods disclosed in JP-A 6-056985, JP-A 10-259151 and JP-A 2002-212335 have a complicated step of separating and collecting a decomposition product and a solvent and produce unwanted by-products.